1. Field of the Invention
The invention relates to ball bonding wire leads between substrates such as lead frames, ball grid arrays (BGA's), or chip carriers and integrated circuit chips, and more particularly to a method and apparatus for supplying a protective cover gas during ball formation to permit the use of wire formed from metals which may react with air, such as copper or aluminum.
2. Description of Related Art
One of the many steps involved in the fabrication of integrated circuit packages is wire bonding of the semi-conductor die contact pads to the lead frame or chip carrier. Particularly, once a semiconductor die has been fully fabricated and diced from the wafer, it is mounted onto a lead frame or chip carrier package. At this point, the contact pads on the surface of the die must be electrically coupled to the pins of the lead frame so that signals may pass to and from the die to external circuitry through the pins on the lead frame. The pins will protrude from the final molded integrated circuit package within which the die will ultimately be encased during fabrication. Generally, each contact pad is electrically coupled to one pin on the lead frame by means of wire bonding.
Wire-bonding machines electrically connect the contact pads on the die to the electrical paths (i.e., pins) on the lead frame by placing a jumper wire which runs from the contact pad on the die surface to the conductive path on the lead frame. In jumper wire bonding, a fine lead or bonding wire is held in a capillary tool so that the lead wire projects beyond the end of the tool. The capillary tool forms part of a ball-bonding machine in which the tool is appropriately mounted and positioned over the metalized die pad of an integrated circuit chip. Descriptions of ball-bonding machines are set forth in U.S. Pat. Nos. 3,643,321; 4,098,447 and 4,323,759, incorporated herein by reference.
In a typical bonding process, the end of the lead wire is formed into a ball of molten metal by, for example, causing an arc discharge between the bonding wire and an electrode which is placed very close to the end of the wire protruding from the capillary tool. This electrode is commonly termed the electrical flame-off or EFO electrode. The melted portion of the metal at the end of the lead wire forms a spherical ball under surface tension. After solidification of at least the surface of the ball, the capillary tool is lowered so that the lead wire is brought into contact with the metalized die pad, or other substrate, and a bond is formed by thermocompression and/or ultrasonic bonding. See, for example, U.S. Pat. Nos. 4,390,771; 4,476,366 and 4,909,427, incorporated herein by reference.
The wire used in such ball-bonding processes may be a non-reactive metal such as gold, or a more reactive metal such as copper, silver, palladium or aluminum. When reactive metals such as copper or aluminum are melted in air, they may react with oxygen to form oxides which interfere with bonding. It is therefore desirable to provide a protective cover gas which does not react with the metal around the molten ball, at least until the surface has solidified and cooled sufficiently to become non-reactive. U.S. Pat. Nos. 4,390,771 and 4,909,427 describe methods and apparatus for providing such a cover gas in which a moveable shroud or shield moves into position before ball formation. The shroud is then filled with a cover gas and the ball is formed at the end of a capillary tool. The shroud is then removed, and the ball-bonding process is completed. Such apparatus require complex movement of the shroud relative to the capillary tool, requiring control equipment and adding steps to the bonding process. Furthermore, the rapid removal of the shroud after ball formation causes a sudden rush of air to impinge on the hot wire ball. The air can cause surface oxidation of reactive metals, as well as uneven cooling of both reactive and non-reactive metals. Furthermore, the open-ended shroud requires a relatively large amount of gas to maintain a cover around the capillary tool and must be refilled at the beginning of each cycle.
A different method and apparatus for providing a cover gas in a ball-bonding process is set forth in U.S. Pat. No. 4,575,602. In this patent, a moveable coaxial open cylindrical shroud is provided around the capillary tube. At the beginning of the ball forming process, the shroud is moved into place surrounding the end of the capillary tool. An electrode is also moved into place below the end of the cylinder and close enough to the capillary tool to form an arc discharge to the wire. The cylinder is then filled with the cover gas and the arc discharge is initiated. After formation of the ball, the electrode is withdrawn and the cylinder is raised to permit the end of the capillary tool to be lowered towards a die pad to complete the ball-bonding process. This process requires precise timing and complex movement of the different components. The open-ended cylinder requires a relatively high continuous flow of gas to maintain the cover during ball formation and must be refilled at the beginning of each operating cycle.
In a fully automated fabrication facility, thousands of dies can be run in a single day comprising tens of thousands of wire bonds. It can be seen that wire-bonding machines are an integral part of a semiconductor fabrication line and that efficient operation of such equipment can have a significant impact on the rate at which integrated circuits chips can be fabricated. Accordingly, it is desirable to ensure that all components of the apparatus work together in a well-coordinated operation.
Accordingly, it is an object of the present invention to provide a method for supplying a cover gas to a ball-bonding process which method is able to work in an efficient and coordinated operation with the other steps of a ball-bonding process.
Another object is to provide apparatus for supplying a cover gas to a ball-bonding process which apparatus is able to work in an efficient and coordinated operation with the other components of a ball-bonding apparatus.
It is a further object of the present invention to provide a method and apparatus which minimize the amount of cover gas required to protect the molten ball formed such ball-bonding processes.